PR libstdc++/86398 fix std::is_trivially_constructible regression

PR libstdc++/86398 fix std::is_trivially_constructible regression

The intrinsic doesn't check for allowed conversions between scalar
types, so restore the std::is_constructible check.

Also make some trivial whitespace changes.

	PR libstdc++/86398
	* include/std/type_traits (is_trivially_constructible): Check
	is_constructible before __is_trivially_constructible.
	* testsuite/20_util/is_trivially_constructible/value.cc: Add more
	tests, including negative cases.
	* testsuite/20_util/make_signed/requirements/typedefs_neg.cc: Use
	zero for dg-error lineno.
	* testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc:
	Likewise.

From-SVN: r262379